1. Field of the Invention
This invention relates to a pit mounted dock leveler which acts as a bridge between a loading dock platform and the bed of a carrier and, more particularly, to a dock leveler having a control system for positioning the dock leveler at selected positions relative to an adjacent loading dock platform.
2. Description of Related Art
Dock levelers for bridging a gap between a loading dock platform and a carrier bed are in widespread use for facilitating the loading and unloading of various types of carriers. Forklift trucks are often required to move from a loading dock directly into and out of the storage compartment of a carrier. In order to facilitate such an operation, dock levelers are used to compensate for any height variation which exists between a loading dock platform and the bed of a carrier.
Typically, a dock leveler will have a main deck which pivots relative to an adjacent loading dock platform and a lip member which pivots relative to the main deck. The lip member will rest directly on the bed of a carrier when the dock leveler is positioned as a bridge. Movement of the main deck and lip member can be achieved by either mechanical means or by fluid pressure. Hydraulic dock levelers are often preferable to mechanical dock levelers due to the decreased amount of physical work involved in positioning the main deck and lip member.
There are two basic operational modes of a dock leveler. The first of these modes is commonly referred to as cycling. During the cycling mode, the dock leveler is moved from a stored position to a bridging position for bridging the gap between a loading dock platform and the bed of a nearby carrier. The preferable stored position for a dock leveler is with the main deck extending substantially level or co-planar with an adjacent loading dock platform so that the dock leveler is in effect, an extension of the loading dock platform. When the dock leveler is in a bridging position, the lip member rests on and overlaps the bed of the carrier so that a forklift may move directly from a loading dock platform across the main deck and over the lip member directly into the storage compartment of a carrier.
The second operational mode of a dock leveler is commonly referred to as recycling. During the recycling mode, the dock leveler is moved from the bridging position to the stored position. It is a dock level recycling mode to which the present invention is particularly directed.
Several prior art systems have been developed to recycle a dock leveler from a bridging position to a stored position. For example, mercury switches, timers, and proximity sensors are well known and widely used to facilitate the recycling operation. Each of these prior art systems senses the position of the main deck during the recycling mode.
The usual method of using mercury switches includes closing the mercury switch when the main deck bottoms out to activate the dock leveler control system for raising the main deck. When the main deck is at specified level above the loading dock platform height, the circuit of the mercury switch is broken to de-activate the dock leveler control system. The dock leveler will then descend to a stored position in which the main deck is level or co-planar with the loading dock platform.
Proximity sensors are used in a similar manner to mercury switches and also rely on the position of the main deck. Typically, a magnetic proximity sensor is used for closing a switch to activate the dock leveler control system. These systems rely on the distance between the main deck and frame to close a switch for raising the main deck and to subsequently open the switch to halt the upward movement of the main deck at a selected level.
Several problems are created by the use of mercury switches and proximity sensors to recycle dock levelers. The first problem involves a threat of serious injury to a forklift operator who is moving across the dock leveler into and out of the storage compartment of a vehicle. Namely, a "false recycle" can occur unbeknownst to a forklift operator who is backing out of a vehicle storage compartment. The main deck will rise above the loading dock platform level and the forklift operator will either fall into the dock leveler or run directly into the lip member. By design, dock leveler main decks will float up and down a limited distance to follow the motion of the carrier suspension system. Downward movement of the main deck is known for accidentally activating both mercury switches and proximity sensors to cause a "false recycle." Such a "false recycle" cannot occur if the recycling operation is responsive to the position of the lip member.
A second problem associated with mercury switches and proximity sensors is also common to timers. That is, a longer amount of time is required to recycle a dock leveler than is desired or necessary. Switches which are responsive to the position of the main deck do not open as soon as a main deck is at the height it will be stored at. Often, these switches do not open until the main deck is at least a foot above loading dock height. This unnecessary motion in exceeding dock platform height and then descending back to dock height to be stored prolongs the recycling time period. In a busy environment such as a loading dock, this unnecessary time delay reduces the volume of material which can be loaded and unloaded in a day and has a direct negative impact on profitability.